Method for forming luminescent screens



y 1962 c. FELDMAN 3,046,154

METHOD FOR FORMING LUMINESCENT SCREENS Original Filed April 19. 1957IlEll TRANSPARENT REFRACTORY GLASS BASE l TRANSPARENT NON-LUMINESCENTREFRACTORY LAYER 31 TRANSPARENT LUMINES'CENT ZINC SILICATE LAYER (GREENLUMINESCENCE) m TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER ILE E l l lI l LASS BASE 2 ON-LUMINESCENT LAYER 3 MINESCENT zmc SILICATE LAYER 4(GREEN) 5 LUM|NESGENT LAYER LUMINESCENT MAGNESIUM SILICATE LAYEF (RED)INVENTOR CHARLES FELDMAN ATTORNEY v opaque powder.

greatly improved. The transparent films reflect no light layer excitingit to luminescence.

United States Patent Ofitice 3,046,154 Patented July 24, 1962 3,046,154METHOD FOR FORMING LUMINESCENT SCREENS Charles 'Feldman, Hollin Hills,Va., assignor to Davohn Corporation, a corporation of Delaware Originalapplication Apr. 19, 1957, Ser. No. 653,867. -Divided and thisapplication Dec. 22, 1959, Ser.; No. 861,731

2 Claims. (Cl. 117-33.5) (Granted under Title 35, US. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government, of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to amethod for forming luminescent screens whichare useful in cathode ray tubes for the production of color images;

This application vis a division of my copcnding application Serial No.653,867, filed April 19, 1957. 7

According to the prior art, opaque white powder have been used forscreens of cathoderay tubes or television picture tubes toproduce'images. These tubes have the disadvantage of not being sharpbecause. of the light: dispersion caused by the opaque powder. They havethe further disadvantage of reflecting incoming light, thus giving poorcontrast between the background and the light producing image.

The tubes therefore fail to provide good daylight viewing. To producecolored images as V in color television, the only practical processbefore the applicant's invention required placing tiny specs of colorproducing phosphors in regulated order all over the view ing screen.-'There have been some attempts at using layers of the opaquephosphorescent powder with each layer 1 being a separate color. It hasbeen realized for some time that chromatic variations in a cathode raytube could be produced by varying the penetration of electrons into ascreen composed of layers of different phosphors. In practice, the useof powdered phosphors informing these multi-layer screens is completelyout of the question. Each grain of the powdered phosphor is about 1 to 3microns in diameter and layers several grains thick must phosphor whichluminescences a different color than phosphor in the first layer. Thecolor seen by the observer will be a combination of these two. colors.This process can be extended for three or more layers.

-L ittle information exists on electron penetration into and throughsolids. The existing theoretical treatments fit only the simplestsystems and few measured values for solids are available. The situationin phosphors is complicated because the exact mechanism of excitation isnot certain. That is, it is diflicult to determine what part of theelectron energy that is lost in collision actually goes to excite theluminescent center in. the solid. However, one may draw the followinggeneral conclusions from the existing published work:

The depth of electron penetration or the range (R) increases as theelectron velocity or energy (E) increases, according to a relation ofthe following equation:

R=CE

where C and n are constants depending on the system involved. Theconstant n is between 1 and 2.

At sufficiently high electron energies, the luminescent cross sectionand hence the brightness starts to decrease. This decrease, of course,takes place only after the electrons have sufiicient energy to penetratethe phosphor. As long as the electrons are completely stopped in thephosphor layer, they give all of their energy to the solid and theluminescent brightness is directly proportional to the energy. s

It is an object of the present invention to provide a method for formingnew luminescent screens which are useful-in cathode ray tubes for theproduction of color by providing a tube which has transparent layers ofphosphorescent film, each layer producing a different color. In thetransparent layers there is no light diffusion by Hence, the sharpnessof the image is and by making the interior ofthe tube light absorbing,there is no reflection and daylight viewing is greatly improved. Byusing transparent layers of different colored phosphorescent film,colored images can be easily and simply produced.

The general 'idea' of a multilayer chromatic screen is that electronsenter the screen and strike the first phosphor This luminescence isviewed through the remaining phosphor layers and glass substrate. If theelectron energy is low and the electron is stopped in this first layer,then the colorviewed will be only that-due to the phosphor in the firstlayer. As the electron energy is increased, the electrons will begin topenetrate to the second layer and excite the second images. It isalso anobject to provide a'method for forming new luminescent screens whichcontain a thin transparent film or layer of luminescence-activated zincsilicate and a thin transparent film or layer of luminescence-activatedmagnesium silicate which differ in the color of the light emitted bythem on excitation by electron bombardment.

The above and other objects will become apparent and the inventionunderstood from a reading of a following description taken inconjunction with the accompanying drawing in which:

FIGURE 1 is a fiowsheet in illustration of the method of the presentinvention, and

FIGURE 2 is a sectional view in illustration of a luminescent screenwhich may be made by the method of the invention. 1

The method of the invention for forming the lumines-- 15, 1961.Laminated screens for the above said purpose may be made by forming ascreen of thin layers on top of one another in the manner described forthe single layer. Each film is fired before another one is placed ontop. In forming these laminated screens,'the lumines-, cent layer whichrequires the highest firing or forming temperature must be placed on thebase first. This is followed by the screen which requires the nexthighest firing temperature and so on until the required number of layersare deposited. This order of application and firing is necessary toprevent deterioration of previously applied layers when subsequentlayers are fired.

Transparent luminescent screens for application in cathode ray tubes canbe obtained by vaporizing zinc silicate or magnesium silicate and aluminescence activator material and depositing a thin film of the vaporson a transparent refractory base, e.g., silica glass such as quartz .1would normally be a refractory glass.

glass and Vycor glass and subjecting the thin film and base to baking inair at a temperature between about 1000 and 1200 C., preferably at about1100 C., for a period of between about 15 minutes and an hour to form aluminescent film of the metal silicate. The baking is conducted untilthe film is activated to luminesce in the color characteristic of theactivator-silicate metal combination thereof. With manganese asactivator, this color is green for zinc silicate films and red formagnesium silicate films.

The initial films for producing the luminescent screens of my inventionare formed by an application of the known vacuum evaporation-depositionprocess. In the application of this process, the metal silicate and theluminescence activator material are vaporized and the vapors condensedon the transparent substrate within a high vacuum, e.g., of the order of10- to 5X mm. Hg, the process being controlled to deposit thin films ofthe order of thickness of about 0.5 to 5 microns on the substrate. Theinitial film on the substrate contains the solid components of the metalsilicate, but not the metal silicate as such. The solid components ofthe metal silicates are the metal or metal oxide, silicon or the siliconoxides. The necessary oxygen for formation of the metal silicate fromthe solid components in the film is supplied from the air in thesubsequent air baking of the film.

The solid components of the metal silicates can be used in place of themetal silicates as starting materials for forming the initial film onthe substrate, for example, Zinc or zinc oxide and silicon or silica forforming a luminescent zinc silicate screen. The vaporization of thesolid components is conducted from separate vessels within the i J highvacuum and controlled so as to deposit the vapors of the solidcomponents on the substrate in the correct proportions for forming themetal silicate therefrom in the subsequent baking of the film in thepresence of =2 oxygen (air).

The activator may be supplied in the films by using commercial activatedzinc or magnesium phosphor powders for making of the films. It may bealso supplied by separately and simultaneously vaporizing the activatormaterial and the metal silicate or its solid components and condensingthe vapors on the substrate within the high vacuum. Manganese is apreferred activator for the production of the luminescent screens of theinvention. However, titanium or other activators can be employed.

In place of forming the film layers directly on top of one another, itis usually desirable to separate the layers by a non-luminescent film.This causes the pure colors to be more pure for given voltages.Likewise, the layers may be separated by non-luminescent conductinglayers which provide a means of controlling the luminescentcolor.

In the luminescent screen of FIGURE 2, a transparent refractory glassbase is shown at 1, having thereon a thin transparent luminescent zincsilicate layer 3, a thin transparent non-luminescent layer 4 and a thintransparent: luminescent magnesium silicate layer 5. The luminescentzincsilicate layer luminesces in one color,green as shown, and theluminescent magnesium silicate. layer in a different color, red asshown, on excitation by a beam of electrons.

Each luminescent layer is deposited and baked one at a time and one ontop of the other on a substrate, which In the baking process, toactivate or form the luminescent layers, each succeeding layer must beof a kind which is formed 0 activated at a lower temperature or at thesame temperature as the preceding layer. Preferably each luminescentlayer is separated from each other and from the substrate .by anon-luminescent transparent layer.

For example, a three layer screenrof zinc silicate, magnesium silicate,and calcium tungstate, each activated by manganese, would'be formed inthe following manner. First the layer of zinc silicate with themanganese is vapor deposited on the transparent base and then baked inan oxygen atmosphere at a temperature of between about 1000 C. and l200C. preferably at about ll0O C. for a period sufficient to form oractivate the layer in the manner described in the said copendingapplication Serial No. 650,958, now Patent No. 2,998,323. This bakingstep shall be called firing. The layer of manganese activated magnesiumsilicate is vapor deposited on top of the zinc silicate layer accordingto the steps outlined in the said copending application Serial No.650,958,

now Patent No. 2,998,323. It is desirable to place two layers ofmagnesium silicate over the zinc silicate. The first layer is fired onlyto a temperature sufiicient to clear the film of magnesium silicatewhich is condensed in a black opaque state. A second layer of magnesiumsilicate is then condensed from the vapors in the vacuum on top of thiscomposite screen. The entire screen isthen baked in an oxygen atmosphereat ll00 C., for a time sufficient to form or activate the magnesiumsilicate luminescent layer. The third layer of tungsten activatedcalcium tungstate is vapor deposited on top of the first two layers andthen baked in air at a temperature of about 550 C., for a timesulficient to form or activate the luminescent film of calcium tungstatein the manner described in the said copending application Serial No.650,958, now Patent No. 2,998,323. A conducting transparent layer isformed on top of the lastevaporated film. This layer may be composed oftin oxide, for example, or formed of a thin metallic film in a wellknown manner. The non-luminescent transparent layers separating theluminescent layers from each other and from the glass substrate areformed by evaporating and depositing a layer of silicon dioxide, forexample, in a well known manner between the evaporating and firing ofthe luminescent layers. Such a layer would be non-conducting. The screenthus .formed is useful in a cathode ray tube to produce colored imagesas in a color television system.

A further use of the screen is an electronic display which appears tohave depth by the stereoscopic effect. A further use is for highresolution electronic display wherein the amount of information is notlimited by optical scattering from particles comprising the screen.

In addition to uses with cathode ray tubes, it is obvious that theluminescent layers can also be excited by other forms of radiant energy,for example, ultraviolet, X-ray,

gamma ray, etc. They also may be excited by an electric field. The thinlight transmitting laminated screen thus can be employed forelectroluminescent illumination, and can be further employed incombination with other films such as photoconductive surfaces for solidstate display devices.

In the screens formed by the method of the invention the order ofoccurrence of the transparent luminescent silicate layers in respect tothe glass base is first the zinc silicate layer and then the magnesiumsilicate layer. This order of occurrence of the luminescent silicatelayers in the screen insures purity of the characteristic color of thelight emitted by the particular luminescent silicate layer onbombardment with electrons. The transparent nonl-uminescent separatinglayer between the zinc and magn'esium silicate luminescent layers doesnot give this efiect'.

While in the above description reference has been made to a specificembodiment of the invention, such 'is" not i .comprises forming a thintransparent non-luminescent,

non-conducting refractory layer on a transparent refractory glass base,condensing the vaporized solid components of zinc silicate and avaporized luminescence .activator on said tnansparentnon-luminescentrefractory layer within a high vacuum to form thereon a thin layercontaining said solid components in the correct proportions for formingthe zinc silicate therefrom on baking of said layer in the presence ofoxygen, subjecting said layer to baking'in the presence of oxygen at atemperature and for a time sufficient to form zinc silicate and'activate the same, forming a thin transparent non-luminescent refractorylayer on said zinc silicate layer, condensing the vaporized solidcomponents of magnesium silicate and a vaporized luminescence activatoron said transparent non-luminescent refractory layer within a highvacuum to form thereon a thin layer containing said solid com ponents inthe correct proportions for forming magnesium silicate therefrom onbaking of said layer in the presence of oxygen and a luminescenceactivator capable of causing the formed magnesium silicate to emit lightof a color different from that emitted by said luminescent zinc silicntewhen excited lay-bombardment with electrons, and subjecting said layerto baking in the presence of oxygen at a temperature and for a timesutficient to form magnesium silicate and activate the same.

2. A method of making a luminescent screen as defined in claim 1,wherein the temperature for baking of the layers containing the solidcomponents of the respective metal silicate is from about 1000 to 1200C.

References Cited in the file'ofthis patent UNITED STATES'PATENTS2,312,229 Anderson Feb. 23, 1943 2,590,018 Koller et al Mar. 18, 19522,600,579 Ruedy et al. June 17, 1952 2,887,401 Cusano May 19, 19592,980,550 Seats Apr. 18, 1961 2,998,323 Feldman Aug. 29, 1961

1. A METHOD OF MAKING A LUMINESCENT SCREEN WHICH COMPRISES FROMING ATHIN TRANSPARENT NON-LUMINESCENT, NON-CONDUCTING REFRACTORY LAYER ON ATRANSPARENT REFRACTORY GLASS BASE, CONDENSING THE VAPORIZED SOLIDCOMPONENTS OF ZINC SILICATE AND A VAPORIZED LUMINENESCENCE ACTIVATOR ONSAID TRANSPARENT NON-LUNINESCENT REFRACTORY LAYER WITHIN A HIGH VACUUMTO FORM THEREON A THIN LAYER CONTAINING SAID SOLID COMPONENTS IN THECORRECT PROPORTIONS FOR FORMING THE ZINC SILILCATE THEREFROM ON BAKINGOF SAID LAYER IN THE PRESENCE OF OXYGEN, SUBJECTING SAID LAYER TO FOR ATIME SUFFICIENT TO FORM ZINC SILICATE AND ACTIVATE THE SAME, FORMING ATHIN TRANSPARENT NON-LUMINESCENT REFRACTORY LAYER ON SAID ZINC SILICATELAYER, CONDENSING THE VAPORIZED SOLID COMPONENTS OF MAGNESIUM SILILCATEAND A VAPORIZED LUNINESCENCE ACTIVATOR ON SAID TRANSPARENTNON-LUMINESCENT REFRACTORY LAYER WITHIN A HIGH VACUUM TO FORM THEREON ATHIN LAYER CONTAINING SAID SOLID COMPONENTS IN THE CORRECT PROPORTIONSFOR FORMING MAGNESIUM SILICATE THEREFROM ON BAKING OF SAID LAYER IN THEPRESENCE OF OXYGEN AND A LUMINESCENCE ACTIVATOR CAPABLE OF CAUSING THEFORMED MAGNESIUM SILICATE TO EMIT LIGHT OF A COLOR DIFFERENT FROM THATEMITTED BY SAID LUMINESCENT ZINC SILICATE WHEN EXCITED BY BOMBARDMENTWITH ELECTRONS, AND SUBJECTING SAID LAYER TO BAKING IN THE PRESENCE OFOXYGEN AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO FORM MAGNESIUMSILICATE AND ACTIVATE THE SAME.